Silica fiber measurement system

ABSTRACT

The system with branched optical fibers provides diagnostics and measurement of static and/or dynamic parameters in structures and structural elements. The system includes a structural material or element having a branched optical fiber embedded therein. The branched optical fiber includes a primary optical fiber segment and at least one secondary optical fiber segment branching therefrom. One or more fiber Bragg grating sensors are arranged on, and are in optical communication with, the primary optical fiber segment and the at least one secondary optical fiber segment. A signal analyzer receives signals generated by the fiber Bragg grating sensors representative of a magnitude of the physical parameter of the structural element.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to diagnostics and measurement of staticand/or dynamic strains in structures, structural elements and structuralmaterials, and particularly to a structural material with branchedoptical fibers embedded in the structural material for measurement ofphysical parameters.

2. Description of the Related Art

The science of determining changes to the material and/or geometricproperties of a structure is referred to as Structural Health Monitoring(SHM). Generally, SHM involves the observation of a structure over timeusing periodically sampled measurements from an array of sensors, andthe analysis of these measurements to determine the current state ofstructural health. There are many different sensors and sensing networksfor accomplishing this, but many have inherent limitations that renderthem unsuitable for certain applications.

One technique that is rapidly gaining in popularity involves fiber opticsensing networks. Optical fiber sensors typically involve a lightpropagating beam which travels along an optical fiber network. Withineach fiber the light is modulated as a function of strain, temperature,bending or other physical or chemical stimuli. The modulation can beanalyzed in either reflection or transmission to determine thecharacteristic of interest. Optical fiber sensors (OFS) have manydistinct advantages including immunity to electromagnetic interference,long lifetime, lightweight, small size, low cost, high sensitivity, etc.Optical fiber sensors (OFS) are typically composed of numerous opticalfibers and numerous Fiber Bragg gratings (FBGs) periodically-spacedalong the length of each fiber. Each FBG creates a periodic variation ofthe optical refractive index in the core of its associated opticalfiber, and when coupled to an interferometer it becomes possible todetect strain individually through change in its resonant wavelength(i.e., the wavelength at which each grating has its maximumreflectance).

“Smart” or “nervous” materials with embedded optical fibers are in arelatively early stage, having the embedded fiber Bragg grating (FBG)array sensors, piezo wires or the like arranged in a relativelyrudimentary and conventional manner. For example, materials with FBGarray sensors arranged in linear stripes and as a regular, rectangulargrid pattern are known. Such arrangements provide models for “proof ofconcept” purposes, but are not well suited to “real world”implementations, where measurements are typically not taken linearlyand/or in regular patterns.

Thus, a system with branched optical fibers addressing theaforementioned problems is desired.

SUMMARY OF THE INVENTION

The system with branched optical fibers provides diagnostics andmeasurement of static and/or dynamic parameters in structures andstructural elements. The parameters are physical parameters, such asstress, strain, deformation, temperature or the like. The systemincludes a structural material having a branched optical fiber embeddedtherein. The branched optical fiber includes a primary optical fibersegment and at least one secondary optical fiber segment branchingtherefrom. A light source, such as a laser, is optically coupled withthe primary optical fiber segment, as is a signal or spectral analyzer.

A plurality of fiber Bragg grating sensors are provided, such that afirst set of the plurality of fiber Bragg grating sensors are arrangedon, and are in optical communication with, the primary optical fibersegment, and a second set of the plurality of fiber Bragg gratingsensors are arranged on, and are in optical communication with, the atleast one secondary optical fiber segment. The signal analyzer is incommunication with the plurality of fiber Bragg grating sensors toreceive signals generated by the fiber Bragg grating sensorsrepresentative of a magnitude of the physical parameter of thestructural element.

These and other features of the present invention will become readilyapparent upon further review of the following specification anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates a system with branched opticalfibers according to the present invention.

FIG. 2A is an exploded view of an exemplary branched optical fiber,illustrating manufacture of a branched optical fiber segment accordingto the present invention.

FIG. 2B is a perspective view of the manufactured branched optical fibersegment of FIG. 2A.

FIG. 3 diagrammatically illustrates an alternative optical fiberbranching arrangement for use with the system with branched opticalfibers.

Similar reference characters denote corresponding features consistentlythroughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The system with branched optical fibers 10 provides diagnostics andmeasurement of static and/or dynamic parameters in structures andstructural elements. The parameters are physical parameters, such asstress, strain, deformation, temperature or the like. As best shown inFIG. 1, the system with branched optical fibers 10 can include astructural element 12. It should be understood that the shape andrelative dimensions of the structural element shown in FIG. 1 are shownfor exemplary purposes only. The structural element 12 may be aluminum,steel, plastic, polyvinyl chloride (PVC) or any other structuralmaterial.

As best seen in FIG. 1, the structural element 12 has a branched opticalfiber 30 embedded therein. In the example of FIG. 1, the branchedoptical fiber 30 includes a primary optical fiber segment 16 a with twosecondary optical fiber segments 16 b, 16 c branching therefrom withinthe bulk of structural element 12. A light source, such as laser 24, isoptically coupled with the primary optical fiber segment 16 a, as is asignal analyzer, which may be a conventional optical spectral analyzer28 or the like. It should be understood that any suitable type ofconventional optical coupler 26 may be used to couple laser 24 andoptical spectral analyzer 28 with the primary optical fiber segment 16a.

The structural element 12 can be any material suitable for forming astructural element. For example, the structural element 12 can be formedfrom thermoplastic or sintered metal materials. The branched opticalfiber 30 may be embedded within structural element 12 by any suitablemethod of fabrication. For example, in the case of plastics, PVC andother thermoplastic or thermomoldable materials, the thermomoldablematerial may be melted, poured into a mold to form the structuralelement and have the branched optical fiber 30 embedded therein withinthe mold. Alternatively, a rapid prototyping machine or the like couldmanufacture structural element 12 with the branched optical fiber 30embedded therein in sequential steps. As another example, for steel,aluminum and the like, the system with branched optical fibers 10 couldbe formed through powder sintering, as described in detail below.

In an embodiment, aluminum powder, for example, can be placed in a dieand the branched optical fiber 30 can be embedded within the powder inthe die. A press can then be used to compress the powder with thebranched optical fiber 30 embedded therein. The compressed structure canthen be sintered in a conventional oven, a microwave oven or the like.Desired positioning of the optical fiber within the material can beaccomplished during the manufacturing process by transmission of lighttherethrough to be received by a receiver for measuring transmittedoptical power.

The optical fiber branch can be formed from a suitable fiber opticmaterial, e.g., silica. The optical fiber branch can be cut to a desiredsize and connected to a main fiber branch or ramification using weldingtechniques, e.g., CO₂ laser. The fiber optic cable can be assembled intoa module by initially placing one end of the cable within the packageadjacent to a laser diode. The laser diode can then be excited to directa light beam through the fiber optic cable. The other end of the opticcable can be coupled to a receiver unit which can determine the amountof optical power transmitted through the fiber. The position of thefiber can be varied until a predetermined optical power is detected bythe receiver unit which corresponds to an optimal alignment position ofthe cable. The fiber can then be removed from the package and the clipplaced on a package substrate. The fiber cable can be re-inserted intothe module and onto the clip at the optimal position. For example, thecable can be adjusted until a maximum optical power is detected toindicate alignment between the cable and the laser diode. The clip canthen be laser welded to the substrate. The fiber optic cable can onceagain be adjusted until the cable is aligned with the diode. The ferruleof the cable can then be laser welded to corners of the clip.

As shown in FIGS. 2A and 2B, the branching of the branched optical fiber30 may be formed via splicing of optical fiber segments. As shown inFIG. 2A, an opening 22 is made through the cladding 20 a of primaryoptical fiber segment 16 a. The size and shape of opening 22 is made tomatch the outer diameter 20 b of secondary optical fiber segment 16 bwhen secondary optical fiber segment 16 b is positioned at the desiredbranching angle with respect to primary optical fiber segment 16 a. Asshown, the cores 18 a, 18 b are in optical communication at the point ofbranching. The two optical fiber segments may then be joined togetherwith a laser weld LW, as shown in FIG. 2B, or the like.

One or more fiber Bragg grating sensors are arranged on, and are inoptical communication with, the primary optical fiber segment and the atleast one secondary optical fiber segment. In the example of FIG. 1, theone or more fiber Bragg grating sensors include first, second and thirdfiber Bragg grating sensor sets 14 a, 14 b and 14 c, such that the fiberBragg grating sensors of the first set 14 a are arranged on, and are inoptical communication with, the primary optical fiber segment 16 a, thefiber Bragg grating sensors of the second set 14 b are arranged on, andare in optical communication with, the second, branched optical fibersegment 16 b, and the fiber Bragg grating sensors of the third set 14 care arranged on, and are in optical communication with, the third,branched optical fiber segment 16 c.

As is well known in the art, a fiber Bragg grating (FBG) is a type ofdistributed Bragg reflector constructed in a short segment of opticalfiber that reflects particular wavelengths of light and transmits allothers. This is achieved by creating a periodic variation in therefractive index of the fiber core, which generates a wavelengthspecific dielectric mirror. A fiber Bragg grating can therefore be usedas an inline optical filter to block certain wavelengths, or as awavelength-specific reflector.

As well as being sensitive to strain, the Bragg wavelength is alsosensitive to temperature. This means that fiber Bragg gratings can beused as sensing elements in optical fiber sensors. In an FBG sensor, themeasurand causes a shift in the Bragg wavelength, Δλ_(B) The relativeshift in the Bragg wavelength, Δλ_(B)/λ_(B), due to an applied strain εand a change in temperature ΔT is approximately given byΔλ_(B)/λ_(B)=C_(S)ε+C_(T)ΔT, or Δλ_(B)/λ_(B)=(1−p_(e))ε+(α_(Λ)+α_(n))ΔT,where C_(S) is the coefficient of strain, which is related to the strainoptic coefficient p_(e), C_(T) is the coefficient of temperature, whichis made up of the thermal expansion coefficient of the optical fiber,α_(Λ), and the thermo-optic coefficient, α_(n). Thus, fiber Bragggratings can then be used as direct sensing elements for strain andtemperature. Fiber Bragg grating sensors for measuring physicalparameters are well known in the art. Examples of such are shown in U.S.Pat. No. 7,702,190 B2; U.S. Pat. No. 7,714,271 B1; U.S. Pat. No.7,973,914 B2; and U.S. Pat. No. 8,705,019 B2, each of which is herebyincorporated by reference in its entirety.

The optical spectral analyzer 28 is in communication with the sets 14 a,14 b, 14 c of fiber Bragg grating sensors to receive signals generatedthereby, which is representative of a magnitude of the physicalparameter of the structural element. As an example, if a stress orstrain is applied to the material 10, a measurement of the magnitude ofthe stress or strain is measured by the plurality of fiber Bragg gratingsensors, and may also be located by comparison of strain magnitudesmeasured by each of the individual sensors.

The extent of branching of the optical fiber 30 is related to theaccuracy and coverage of the sensors embedded within structural element12. It should be understood that the simple forked structure shown inFIG. 1 is shown for exemplary purposes only. Preferably, multiplelevels, or layers, of branching are provided, allowing for greateraccuracy and coverage of sensing within the material. As an example, theoptical fiber 30 of FIG. 1 is expanded upon the exemplary configurationof FIG. 3. In FIG. 3, optical fiber 30 includes a primary optical fibersegment 16 a which branches into optical fiber segments 16 b and 16 c,as in FIG. 1. However, a further optical fiber segment 16 d branches offof optical fiber segment 16 b. Similarly, a further optical fibersegment 16 f branches from optical fiber segment 16 c, and optical fibersegment 16 e additionally branches from primary optical fiber segment 16a. Each new segment includes its own corresponding fiber Bragg gratingsensors 14 d, 14 f, 14 e, respectively. It should be understood that theconfiguration shown in FIG. 3 is shown for exemplary purposes only.Further, it should be understood that each of the new branches of FIG. 3could, in turn, be further branched. It should be further understoodthat the branching is preferably three-dimensional, allowing sensormeasurements to be made throughout the entire volume of the material.

It is to be understood that the present invention is not limited to theembodiments described above, but encompasses any and all embodimentswithin the scope of the following claims.

1. A silica fiber measurement system with branched optical fibers,comprising: a structural element having a branched optical fiberembedded therein, the branched optical fiber including a primary opticalfiber segment and at least one secondary optical fiber segment branchingtherefrom, wherein the primary optical fiber segment and the at leastone secondary optical fiber segment are spliced to one another such thatrespective cores thereof are in optical communication with one anotherat a point of branching and wherein the secondary optical fiber is madeof silica; a laser optically coupled with the primary optical fibersegment; one or more fiber Bragg grating sensors on and in opticalcommunication with the primary optical fiber segment and the at leastone secondary optical fiber segment; and an optical spectral analyzer incommunication with the plurality of fiber Bragg grating sensors, theoptical spectral analyzer receiving signals generated by the fiber Bragggrating sensors representative of a magnitude of a physical parameter ofthe structural element.
 2. The silica fiber measurement system withbranched optical fibers as recited in claim 1, wherein the physicalparameter of the structural element includes at least one of strain,stress, deformation and temperature.
 3. The silica fiber measurementsystem with branched optical fibers as recited in claim 1, wherein theat least one secondary optical fiber segment has at least one tertiaryoptical fiber segment branching therefrom.
 4. The silica fibermeasurement system with branched optical fibers as recited in claim 1,wherein the structural element includes a thermoplastic or metalmaterial.
 5. The silica fiber measurement system with branched opticalfibers as recited in claim 4, wherein the structural element includes asintered metal material.
 6. The silica fiber measurement system withbranched optical fibers as recited in claim 1, wherein the one or morefiber Bragg grating sensors includes a plurality of fiber Bragg gratingsensors, the plurality of fiber Bragg grating sensors including a firstset of the plurality of fiber Bragg grating sensors arranged on and inoptical communication with the primary optical fiber segment, and asecond set of the plurality of fiber Bragg grating sensors arranged on,and in optical communication with the at least one secondary opticalfiber segment. 7-9. (canceled)